GUTTATIOX 77 



temperature and dryness of the surrounding air, sun- 

 shine, etc. 



115. Many plants exude water from specially modified 

 stomata (the so-called water pores) at the edges of the 

 leaves when the movement of water upward has been 

 strong and then, by increase of the humidity of the air, 

 the evaporation has been checked rather suddenly. 

 This may take place in the form of drops or even as a 

 fine stream. It is called guttation. Its mechanics and 

 use are not clear. 



Laboratory Exercises. Note : In a large class, many of these 

 experiments cannot be performed by every student. In that 

 case the instructor should assign some experiments to one 

 student, others to another throughout the class or should set 

 up the experiments himself before the class. In either case, 

 every student should make complete notes upon the experiment 

 for himself. 



(a) Weigh a handful of freshly picked leaves quickly before 

 they have begun to wilt. Place them in an oven at the 

 temperature of about 110° C. and dry them for twelve to 

 twenty-four hours. Now weigh them and note the loss in 

 weight. This is almost entirely due to the evaporation of the 

 water in the leaf. Calculate the percentage of water in the 

 original weight. Repeat the experiment with various parts of 

 the same plant such as stems, roots, flowers, fruit, seeds, etc., 

 and compare the amount of water in these different parts as 

 well as with the corresponding parts of other plants. 



(b) To demonstrate imbibition by cell walls, take a measured 

 block of wood 5 or 6 cm. long and 3 or 4 cm. square. Measure 

 it when perfectly dry, i.e. after having been kept a day or two in 

 an oven at 110° C. Then soak it in water (preferably warm or 

 hot, to hasten the process). Now measure accurately. The 

 piece will be found to have become perceptibly larger owing to 

 the imbibition of water by the cell walls. Probably the first 

 entrance of water into dry seeds is also due to imbibition of 

 water by the cell walls and protoplasm. As soon, however, as 

 the latter has imbibed enough to become hquid, osmosis 

 begins to act also in the taking in of water. 



